More robust and easier to service than any other tractor in its day and age

The OE agricultural tractor chugged into the limelight at the 1928 International Motor Show (IAMA) and on to its buyer in May that year. It was vastly superior to its contemporary competitors in terms of economic efficiency, but in engineering terms, too, it was one of the most matured designs of the 1920s. The OE incorporated all the experience gained in decades of engine and agricultural machinery development.

As early as 1902, Daimler had been awarded a first prize from Deutsche Landwirtschafts-Gesellschaft (DLG, German Agricultural Association) for its locomobile. However, all-out suitability for field work was not achieved before 1913 when Daimler introduced a motorized plow. This articulated plow weighed 6.6 tons and was not so far away from the principle of tractor-plus-trailing-plow, which was to prevail in the 1920s. In 1921, finally, Daimler presented such a plow tractor, over five meters long and weighing four tons.

The world’s first vehicle diesel engine to power a tractor

After the merger with Benz in 1926, however, everything spoke in favor of the former competitor’s model. The decisive criterion was the diesel engine which had been installed in a three-wheeled tractor by Benz as early as 1922. Benz had developed this somewhat weird-looking tractor with a single, roller-type drive wheel with a diameter of 1.40 meters immediately after World War I, together with Munich-based engine and tractor producer Sendling. Its heavy-oil engine was really only used to test the pre-chamber diesel in practical operation, one year before the latter was built into a truck for the first time. But the diesel-engined tractor became a complete success, too.

The prototype was sold on the spot, at the agricultural show in Königsberg in 1922. Two additional units followed before series production of the diesel-engined Benz-Sendling S6 tractor with a single drive wheel started one year later. By 1925 the first one hundred tractors of this type had been sold, followed by another 200. Overall, Benz-Sendling was able to sell 1,188 units of the three-wheeled tractor until the early thirties.

The main reason for the plow’s design with a single drive wheel – or that of Daimler’s very similar plow tractor with two rear wheels arranged very close to each other – was the fact that it did not require a differential. Very soon, however, it was realized that this advantage had been achieved at the expense of stability: the tractor was prone to toppling over. For this reason, Benz-Sendling developed the four-wheeled diesel-engined BK tractor as early as 1923, and this model was also available as a road-going version with solid rubber tires. It was the direct predecessor of the OE.

Coordination problems after the merger

Buyers do not always decide on the grounds of technical sensibility, however. While the three-wheelers sold like hot cakes, their four-wheeled brethren met with a good deal of reservation on the part of buyers. It was to hand down this fate to its successor, the OE model, and to make things worse, the latter was introduced at an unfavorable point in time.

After the merger in 1926, Daimler and Benz were preoccupied with newly organizing their different business activities. During the later stages, Benz-Sendling had had its tractors built by Komnick in East Prussia – hence the designation BK: a Benz diesel engine in a Komnick chassis. The OE, by contrast, was to be produced in Stuttgart-Untertürkheim. By the time everything was ready for production, two years had passed and soon afterwards, the world economic crisis was to reduce the customers’ purchasing power quite severely.

Hotly contested market

It wasn’t exactly easy to achieve success with a new tractor in the German market of the 1920s. As many as 70 companies competed in this field around 1920. Ford’s Fordson tractor, built in large series and being significantly cheaper, threatened to push all others out of the market. When import restrictions could no longer be maintained in 1924, the German government resorted to loans specifically granted to buyers of German products. Several companies offered highly favorable payment conditions or compared their products directly with the Ford tractor in aggressive advertising campaigns, naturally presenting their own tractors in the most favorable light.

This somewhat crude approach was not Daimler-Benz’s style. The company drew up comparisons indirectly at the most. In a brochure for the 1928 IAMA, the silhouettes of two differently sized tractors illustrated copy with the headline “Small tractor?” – quite enough to tell readers what it was all about. The copy itself explained that “however powerful an engine may be, it can only pass on so much output to the tractor’s towing hook as the driven wheels are capable of transmitting without digging themselves into the ground. Under identical ground conditions, the so-called slip at the driven wheels depends on the size, grab angles and weight of the tires.”

The rear wheels on the diesel-engined Mercedes-Benz tractor had a diameter of 1.30 meters; they were made of flat-iron sections with 14 diagonally arranged grab angles. The deep-tread rubber tires of modern tractors were still unheard of at the time. Tipping the scales at some 2.5 tons, the OE was a ton heavier than the Ford lightweight. “The higher weight prevents the machine from rearing or rolling over under any circumstances” – that’s how the brochure discreetly referred to yet another important difference.

At 24 hp (18 kW) and, at a later stage, 26 hp (19 kW), the OE even was less powerful than the 30 hp (22 kW) Fordson. That, however, was not important; what counted was the tractive power when starting off, especially on wet or sandy soil or on uphill gradients. Let’s quote the brochure again: “In practical operation, the vehicle has the same performance as a so-called small tractor with an output in excess of 30 hp. The frequently observed dangerous rearing and rolling over of small tractors is reliably prevented by the high front-axle pressure of over 1,000 kilograms.”

State of the art: Frameless block design, heavy-oil engines, flywheels

However, the OE’s design was far from oriented to its American competitor. In Germany, two tractors were capable of holding their own against the Fordson, and these two were produced on assembly lines from the mid-1920s. One was the WD from Hanomag, a direct competitor in the lower weight category, low-priced and with technical improvements. The other was the OE from Mercedes-Benz, which was more like the Bulldog from Lanz.

There was one thing the German tractor manufacturers had learnt from Ford: frameless block design, with engine block, transmission housing and rear axle forming a torsionally rigid entity – a clearly superior design to previous frame constructions in coping with the high loads encountered when plowing rough fields. This – as well as the economic advantages of heavy oil – had been realized by Lanz as early as 1921. However, the Bulldog was not powered by a diesel engine but by a hot-bulb heavy-oil engine patented by Herbert Akroyd Stuart and further developed by the company’s chief engineer, Fritz Huber. The OE largely corresponded to the Bulldog in terms of dimensions, weight, engine output, speed and reduction in the three-speed transmission. Laterally arranged flywheels with one of them containing the clutch, as on the Lanz vehicle, also provided for similarity in terms of looks.

While Lanz’s hot-bulb engine was considered economical given the low crude-oil prices, the Daimler-Benz diesel engine was a straightforward miracle in terms of economic efficiency. This was the main reason that prompted the Untertürkheim-based company to send the OE out into the field to compete against the eleven remaining German tractor producers in 1928. However, Daimler-Benz was not the only one to launch a new tractor: Deutz had introduced its diesel-engined MT 222 tractor somewhat earlier, though initially only in a road-going version with rubber tires.

Cornering radius of just four meters

It has to be said, however, that the OE was far more than just a diesel-engined copy of the Lanz Bulldog. In quite a number of technical features, the tractor broke new ground, thereby contributing significantly toward making farm work easier. This began with the careful tuning of power unit and power transmission; it continued with the weight distribution to front and rear axles and did not end with the size and width of the wheels which could be broadened for operation on marshy soil.

A decompression unit facilitated the starting of the engine by means of an ignition capsule. A control element on the camshaft, reducing revolving speed as load declined and vice versa, proved to be an ingenious invention. It ensured that the highly flexible, horizontally installed single-cylinder engine evenly transmitted its power to the ground at all times, even when starting on soft soil, under changing loads and in mountainous terrain. A “rising torque at declining engine speed” was also praised as an advantage of the Mercedes tractor in a brochure.

For a tractor of this size, its cornering radius of just four meters was unparalleled. It goes without saying that this required a differential. Not all the tractors, however, had a differential lock to prevent wheel spin. The differential lock was designed so as to convert the two halves of the rear axle into a rigid unit – to prevent the different loads of cornering wheels from being transmitted back to the gearbox in case the farmer forgot to release the differential lock again. A pendulum-type suspension on the front axle additionally helped in avoiding unnecessary stresses.

The characteristic front view of the OE revealed four major elements: the single-cylinder engine mounted at the front of the transmission housing, a flat rectangular receptacle arranged above the engine and containing coolant and fuel tanks, a high-rising air filter on the right-hand side and the exhaust on the left. The low heat generated by the diesel engine permitted the use of evaporation cooling instead of recirculation cooling, conventional at the time but prone to freezing. Nevertheless, Daimler-Benz offered recirculation cooling for tractors exported to warmer countries.

The efficient, long-term force-feed lubrication also benefited from the close combination of engine and gearbox in a single, torsionally rigid housing. In an article for the trade journal “Die Technik in der Landwirtschaft” (Agricultural Engineering), Benno R. Dierfeld, a high-ranking government official, wrote in 1929: “The lubrication system, which is crucial for the service life and operational reliability of the engine, is designed with outstanding care.” Compared to chain, pinion or worm-gear drive, the direct transmission of power from the gearbox to the rear wheels via spur gears proved to be virtually indestructible.

Thanks to the easiest-conceivable handling, well-matched parts and an extremely robust design, the OE was a reliable workhorse, capable of easily coping with the most arduous conditions. In spite of this, Daimler-Benz had taken unique precautions against signs of wear: two large, easily removable maintenance flaps provided easy access to the dust-tight transmission housing, described by the brochure as “a unique and valuable new feature, not to be found on any other manufacturer’s product.” It was equally easy to replace the cylinder liner, made of a special iron alloy with chromium and nickel additives, in case it was found to be worn out after a large number of operating hours.

Jack-of-all-trades

Earlier motorized plows had had the big disadvantage of only being suitable for heavy-duty plowing work due to the rigid connection of plow and drive unit. The versatile OE tractor, by contrast, was capable of doing a lot more, as revealed by the table of performance figures in the tractor brochures: in deep plowing, the tractor covered between 3.75 and five acres in a ten-hour working day; it managed between 6.25 and 7.5 acres in sowing and plowing with a two- or three-share plow and between 12.3 and 15.5 acres in skim plowing with a five-share top-soil plow. And it was able to cultivate and harrow up to 25 acres per day. Harvesters, potato or beet lifters and sorters were driven via a power-take-off. In addition, it was possible to mount a belt pulley whose revolving speed could be adjusted to anything between 300 and 800 rpm to drive stationary machinery. In this way, the tractor could also be used to drive pumps, circular saws, threshers and chaff cutters, grinding mills and even dynamos.

The farm tractor was also available in a road-going version, its grab-angle wheels replaced by all-metal wheels with solid rubber or pneumatic tires and a different transmission ratio, achieved by the replacement of two gearwheels. While the farm tractor reached 3.2 km/h in first gear, 4.5 km/h in second and 6.2 km/h in third gear, the road-going tractor managed 10.6 or 12.3 km/h, depending on the trailer load, and as much as 15 km/h at a later stage. Customers were able to choose: the standard tractor pulled 15 tons in third gear on a level road – for carrying higher loads or climbing slopes, it was possible to change the overall ratio accordingly.

The road-going tractor featured a simpler air filter version. A belt pulley for driving stationary machinery was optionally available at extra cost, and the same applied to a rear-mounted rope winch, a soft-top with sides to protect the driver from inclement weather, as well as petroleum, carbide or electrical lamps. Thanks to maintenance flaps in the transmission housing, the farm tractor was easily converted into a road-going vehicle when required: all that needed to be done was a change of wheels and of two gearwheels in the transmission. At a later stage, Daimler-Benz offered a “combination tractor” version with two sets of wheels and gearwheels for the changeover from field work to operation on the road.

Operating costs making all the difference

Right from the start, Daimler-Benz praised the OE as “the most reliable and cheapest tractor for agriculture” and listed the fuel costs incurred by the individual types of work: between 33 Pfennigs for cultivating and harrowing and 133 – 150 Pfennigs for deep plowing per acre had to be paid by the farmer. The considerably lower price of diesel fuel combined with a fuel consumption that was 30 percent below the gasoline engine’s so that costs could be cut by 1,500 Reichsmarks with 1,200 operating hours per year.

“The return on 1,000 Marks more or less invested capital is virtually insignificant,” it said in a 1928 brochure – a direct reference to the Lanz Bulldog which cost 5,600 Marks, precisely 1,000 Marks less than the OE. Daimler-Benz nevertheless reduced the price to 5,900 Marks somewhat later.

When customers did not immediately realize the cost advantage arising from lower operating costs, the manufacturer went to extremes in directing their attention to the cost-cutting potential. Brochures going into all details of tractor operation costs were printed with two-color covers, revealing at first glance what it was all about: “74 percent reduction in fuel costs!” The unparalleled economy of the tractor was demonstrated time and again at promotion events and in comparative tests, for instance in an international tractor competition in England in 1930 from which the OE emerged as the best performer of all candidates.

A 1931 brochure put it quite bluntly: “Your expenditure on means of transport is too high and has to be reduced. This is achieved by the acquisition of a Mercedes-Benz diesel tractor whose cheap purchase price, lowest operating and maintenance costs and significantly raised performance account for annual savings of several thousand Marks compared to the operation of equally powerful vehicles with gasoline/benzene engines.”

Convincing in practical operation

The company’s PR efforts also extended to obtaining testimonials from buyers of the diesel tractor. The numerous letters Daimler-Benz received from customers from the tractor’s first year onward provided proof of the very high level of customer satisfaction, as well as of the wide range of different uses to which the diesel tractor was put.

There is for instance a letter written by Fritz Teuscher, retired major and owner of the Paunsdorf estate near Leipzig: “In response to your inquiry, I take the liberty of informing you that I am very satisfied with the tractor you supplied in July 1928. In particular, I would like to stress the machine’s great power reserves, permitting the vehicle to start off effortlessly even under the most difficult conditions.” After detailed descriptions of the tractor’s applications and fuel consumption, the major came to the following conclusion: “I can recommend the acquisition of a Mercedes-Benz tractor without any reservations.”

The steward of the city of Speyer’s estate came to a similar conclusion: “We worked with different tractors during the last five years. The diesel tractor outclasses them all in terms of both operating cost savings and ease of operation, and it is in no way superior in terms of tractive power.” The steward, a Mr. Ott, also praised the differential lock and finished his letter with the following comment: “Quite apart from that, I congratulate you on the fact that the famous Benz factory is now at last offering a tractor to German farmers that is superior to all foreign brands, including in our opinion the well-known I.H.C tractor, in every respect and especially in operating cost savings.”

The owner of the Moisall estate, Baron von Swieykowski-Trzaska, equally confirmed “that your Mercedes-Benz diesel tractor performed outstandingly in trial plowing on my estate. Fuel consumption was the lowest of all, and the furrow plowed by your tractor was significantly better than the furrows ploughed by your competitors’ machinery at the same time. This is all the more praiseworthy when you consider that the Mercedes-Benz diesel tractor had to work on extremely mountainous, loamy and stony terrain.”

The majority of testimonials were, however, sent in by large privately or publicly owned estates, one of them even by the “Compagnie Ouest-Cameroun” whose director wrote on June 19, 1930: “With great conviction, I am able to confirm that your diesel tractor machines are of remarkable efficiency and high value, especially in this country where tractor work bears no comparison with European requirements at all. Here, heavy-duty work has to be performed in reclaiming land overgrown with extremely dense and tenacious prairie grass.”

Daimler-Benz also received ample feedback from various industrial and trading enterprises which used the road-going tractor version for a variety of purposes. Among them were coal dealers, haulage companies and breweries, a sugar factory, an iron foundry, a farm machinery producer and an electromechanical workshop. A gravel pit used the tractor as a stone crusher, and a construction company, a plasterer's workshop and a cement and construction materials trader among the testimonial writers testify to the tractor's use in the construction industry. And finally, there was a consortium which used the OE in the construction of a hydroelectric plant in Haute Provence.

Sales lagging behind expectations

The diesel-engined OE truck had everything it takes to become a great success. It was, after all, an extremely robust vehicle, its design well thought out down to the smallest detail, and one that set new standards in maximum ease of maintenance and fuel economy. Moreover, the diesel-engined Benz-Sendling tractor with a single drive wheel – the world's first diesel tractor – had been such a success that its successor, a much better vehicle in many respects, gave rise to great – and perfectly justified – hopes.

And yet, despite the customers' unanimously positive response, the Mercedes-Benz tractor didn't sell well. From today's perspective, the reasons cannot be made out with absolute certainty. The disappointing start is nevertheless likely to be attributable to the unfavorable point in time at which the tractor came onto the market.

Many potential buyers probably reacted somewhat hesitantly to yet another new tractor after so many brands had caused more anger than satisfaction during the first half of the 1920s. The diesel-engined Benz-Sendling tractors had been an exception to this rule but the customers had as yet to grasp the fact that the OE came from the same manufacturer, despite a different production plant and different brand name. Daimler-Benz had therefore been clever enough to announce the OE as a Benz-Sendling product in 1928. The location of Benz-Sendling's partner Komnick near the large Prussian estates may also have contributed to their joint tractor's success. Whereas 888 units of the three-wheeled S7 were sold between 1925 and 1931, no more than 380 customers had by 1935 opted for the OE from Daimler-Benz.

To make things worse, one year after the OE tractor's launch, the world economic crisis shattered many hopes. However low the operating costs, however easy the maintenance, for a normal farmer a purchase price of (eventually) 5,900 Reichsmarks was a steep hurdle – hence the large number of testimonials from large-estate owners and tenants who bought the farm tractor, and the large number of companies which used the road-going tractor for a variety of purposes. But the heyday in farm mechanization was yet to come.

In 1933, when the OE's share in new tractor registrations had reach an all-time low, Daimler-Benz decided to discontinue production, especially since the demand for the company's other products was high. It was after World War II that the company started producing farm tractors again, first in 1951 with the Unimog and then in 1972 with the MB-trac.